If so, I want to confirm with you: If you create a new room mailbox, whether is this phenomenon occurs on it?
I have yet to try

If this phenomenon occurs on new created room mailbox, it means this phenomenon may related with the way that you checked room mailbox or free/busy. -
What do you mean?

If this phenomenon doesn't occur on new created room mailbox, it mean that this phenomenon may related with the configuration on your old room mailbox. What
do you mean? There wasn't much configuration on the 3 room mailboxes.

If so, I want to confirm with you: If you create a new room mailbox, whether is this phenomenon occurs on it?
I have yet to try

If this phenomenon occurs on new created room mailbox, it means this phenomenon may related with the way that you checked room mailbox or free/busy. -
What do you mean?

If this phenomenon doesn't occur on new created room mailbox, it mean that this phenomenon may related with the configuration on your old room mailbox. What
do you mean? There wasn't much configuration on the 3 room mailboxes.

Do you have any solution to fix this issue?

Does this phenomenon occurs on new created room mailbox? It could help us to narrow down this phenomenon?

Regards,

Kyle Xu

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